no slide titlecurrent gain • the q point is ... • the base is n-type material ... no slide title...

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MALVINO & BATES

SEVENTH EDITION

ElectronicPRINCIPLES


Transistor BiasingTransistor Biasing

ChapterChapter 88


Topics Covered in Chapter 8

• Voltage-divider bias• Accurate VDB analysis• VDB load line and Q point• Two-supply emitter bias• Other types of bias• Troubleshooting• PNP transistors


Voltage divider bias

• Base circuit contains a voltage divider• Most widely used• Known as VDB


+VCC

RC

RE

R1

R2

Voltage divider bias circuit

R1 and R2 form a voltage divider


+VCC

R1

R2

+VBB

Divider analysis:

VBB =R2

R1 + R2VCC

ASSUMPTION: The basecurrent is normally much

smaller than the divider current.


IE =VBB - VBE

RE

VC = VCC - ICRC

VCE = VC - VE

IC ≅ IE

+VCC

RC

REVBB

To complete the analysis:

Now the circuit can be viewed this way:


The six-step process

1. Calculate the base voltage using the voltage divider equation.

2. Subtract 0.7 V to get the emitter voltage.3. Divide by emitter resistance to get the

emitter current.4. Determine the drop across the collector

resistor.


The six-step process(Continued)

5. Calculate the collector voltage by subtracting the voltage across the collector resistor from VCC.

6. Calculate the collector-emitter voltage by subtracting the emitter voltage from the collector voltage.


VDB analysis

• The base current must be much smallerthan current through the divider

• With the base voltage constant, the circuit produces a stable Q point under varying operational conditions


+VCC

RC

RE

R1

R2

Is the divider a stiff source?

Find the Theveninresistance.

RTH = R1 R2


+VCC

RC

REVBB

RTH

A Thevenin model of the bias circuit:


+VCC

RC

RE

The 100:1 rule applied to the bias circuit:

RIN

VBB

RTH

RTH < 0.01 RIN

When this rule is met, the divider is stiff.


Firm voltage divider

• Used because divider resistors (e.g. R1and R2) in a stiff design would be too small

• The collector current will be about 10%lower than the stiff value


+VCC

RC

RE

R1

R2

Sometimes a firm divider is chosen.

R1 R2< 0.1 βdcRE

A closer approximation:

IE =VBB - VBE

RE + βdc

R1 R2


VDB load line and Q point

• VDB is derived from emitter bias• The Q point is immune to changes in

current gain• The Q point is moved by varying the

emitter resistor


Two-supply stiff emitter bias:10 V3.6 kΩ

2.7 kΩ 1 kΩ

2 V

Assume 0 V

IE =VEE - 0.7 V

RE

IE =2 V - 0.7 V

1 kΩ= 1.3 mA


10 V3.6 kΩ

2.7 kΩ 1 kΩ

2 V

VC = 10 V - (1.3 mA)(3.6 kΩ) = 5.32 V

VCE = 5.32 V - (-0.7 V) = 6.02 V

Find the voltages:


+VCC

RCRB

Base bias:

•The least predictable•Q point moves with replacement•Q point moves with temperature•Not practical


+VCC

RC

RE

RBEmitter-feedback bias:

•Better than base bias•Q point still moves•Not popular


+VCC

RCRB

Collector-feedback bias:

•Better than emitter-feedback bias•Q point still moves•Some applications because of circuitsimplicity


+VCC

RCRB

RE

Collector- and emitter-feedback bias:

•Better than emitter-feedback bias•Not as good as voltage-divider bias•Limited application


Two-supply emitter bias:

•Very stable•Requires 2 supplies

Note: Also called TSEB


+VCC

RC

RE

R1

R2

Voltage divider bias:

•Very stable•Requires 1 supply•The most popular

Note: Also called VDB


Troubleshooting

• Voltage measurements• Use 10 MΩ input voltmeter• Troubles include:

OpensShortsFaulty transistors


PNP transistor

• The base is n-type material• The collector and emitter are p-type

material• The emitter arrow points in• Can be used with a negative power

supply


Electron flow Conventional flow

ICIB

IE

ICIB

IE

PNP transistor symboland current flow


-VCC

RC

RE

R1

R2

PNP Biasing with anegative supply


+VEE

RE

RC

R2

R1

PNP Biasing with apositive supply

no slide titlecurrent gain • the q point is ... • the base is n-type material ... no slide title...

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